Move infra-setup and k3s-setup into dedicated directories

This changes makes it easier to differentiate and understand the
different parts of the kubernetes setup. On one hand we have the bare
infrastructure (servers, network, etc), on the other hand we have the
software (k3s in this case).

In the future we'll have a few more parts, like the minimal
configuration of the kubernetes cluster, e.g. with a cert-manager. This
is easier to manage with helm or terraform than with ansible. Therefore
it makes even more sense to split the responsibilities into dedicated
directories.

infra/inventory.ini.tftpl

@@ -0,0 +1,13 @@
+[server]
+%{for ip in server_ips~}
+${ip}
+%{endfor~}
+
+[agent]
+%{for ip in agent_ips~}
+${ip}
+%{endfor~}
+
+[k3s_cluster:children]
+server
+agent